The present invention is directed to a method and apparatus for permitting use of a dedicated electrical circuit extending between a tractor and a trailer and used to power the trailer's ABS brake control circuit for purposes of providing auxiliary power whenever the trailer is parked and there is no need for the dedication of the electrical circuit to the ABS control circuit. Further, the present invention is also directed to a failsafe method and apparatus for dedicating an electrical circuit to the ABS brake control circuit by terminating power to any auxiliary circuits connected to an electrical input to the ABS control circuit in response to a buildup of pressure in the vehicle's emergency brake air supply system. With the invention, whenever pressure has been increased in the emergency brake air supply system to release the trailer air brakes, a normally closed pneumatic switch automatically opens the auxiliary electrical circuit and ensures a dedicated "hot" wire circuit directly from the tractor to the trailer ABS control circuit.
Electrical power to a trailer pulled by a tractor is provided by connecting the tractor electrical power supply to a number of dedicated electrical circuits associated with the trailer. Conventionally, the trailer includes dedicated circuits for the left-turn signal, right-turn signal, signal markers, taillights, stoplights, the automatic brake system (ABS) control circuit and ground. Tractor-trailer fleets have conventionally used the circuit adapted for use for powering the ABS control circuit for auxiliary applications, such as for providing power to interior lights, whenever the ABS control circuit is not to be utilized, such as when the trailer is parked. However, it is necessary to ensure that the power to the ABS control circuit is available and protected from excess loads which may cause a loss of power to the ABS whenever the trailer is to be moved. Pending federal guidelines will soon require that the trailer be wired "hot" in order to provide continuous power to the trailer ABS control circuit to ensure proper functioning of the computer governing the trailer braking system whenever the trailer is being moved by a tractor.
Conventional trailers are also mandated to receive braking power from the air brake system of a tractor. In this respect, federal guidelines require that the emergency brakes on the trailer be applied whenever there is a decrease in air pressure to a predetermined level in the emergency brake air lines of a tractor-trailer vehicle. Thus, before a trailer can be moved, it is necessary to either manually override the emergency brakes to allow towing of the trailer by an emergency vehicle or to provide sufficient air pressure within the emergency brake lines to release the emergency brakes.